The invention relates to a method for providing a curved cooling channel in a gas turbine component. The invention furthermore relates to a coolable blade for a gas turbine component having at least one curved cooling channel.
In general, cooling channels in gas turbine components are designed in the form of open passageways that extend along straight line axes. In a number of applications, such as, for example, gas turbine blades subject to high thermal loads and having complex geometries, it is difficult to form cooling channels in a suitable manner for especially highly loaded sections. Points on the blades that are hard to access, such as, for example, in the transition area between the blade hub and the platform or in wall sections with high thermal loads, may require cooling channels having axes that do not extend linearly. A cooling channel that curves three-dimensionally in space would have to be provided in such locations.
European publication EP 0 659 978 A1 discloses a coolable turbine blade that is constructed in a known manner to include a blade hub, a blade root, and a platform. The blade hub includes a suction-side wall and a pressure-side wall that are connected with each other along a leading edge and a trailing edge, with a cavity formed in between the suction-side wall and the pressure-side wall. Curved channels are provided in the region of the blade tip.
With respect to providing the curved channels, general reference is made to electrochemical processes and also to laser beam drilling. However, European publication EP 0 659 978 A1 does not provide further details related to this. The curved channels shown in this publication have also been positioned in the region of the blade tip. Starting from the pressure side of the blade, the curved channels extend to the blade tip. Accordingly, the area where the curved channels are formed is easily accessible, and the curved channels can be formed without great difficulty. Difficulties are encountered, however, when attempting to form curved channels in areas of a gas turbine blade that are hard to access.
In view of the above-discussed difficulties, the invention is directed to a method for forming curved cooling channels, even in regions of the blades that are hard to access. The invention also is directed to a coolable blade for a gas turbine component, in particular a turbine blade, that is provided with a curved cooling channel that enables the required heat removal even at positions on the blade that are hard to access.
According to an embodiment of the invention, a method for forming the curved cooling channels in regions of the blade that are hard to access utilizes an electrode in the form of a helix. The electrode is driven during the formation of the cooling channel so as to rotate around its central rotational axis, resulting in a curved channel in a helical shape. This makes it possible to provide in a simple manner cooling channels in areas subjected to high thermal loads, especially at the transition areas between the blade platform and the blade hub, or in the wall of the blade hub.
The electrode is positioned axially movable in relation to the rotational axis so as to permit the corresponding advance of the electrode according to the pitch of the helix. The electrode is preferably driven in a forcibly coupled manner, moving axially and rotating relative to the rotational axis. This ensures that the electrode is guided optimally in the respective channel section that was just created.
The method according to the invention allows for the selection of a variety of cross-sectional shapes for the electrode wire to form cooling channels having any desired cross section. Examples of cooling channel cross-sections can include rectangular cross-sections, circular cross-sections and ellipsoid cross-sections, with the cross-section chosen to ensure optimum flow conditions within the cooling channel.
Even though the method described above can be used in practically all gas turbine components, it is used preferably in coolable blades.
A coolable blade constructed according to an embodiment of the invention preferably includes a cooling channel with at least one section having a helical shape. Such cooling channels enable an extremely efficient cooling of the blade, especially in regions that are subject to especially strong thermal stresses. The cooling channels according to the invention can be provided in regions of the blade that are hard to access, such as the transition area from blade hub to platform or in wall areas of the blade hub that are subject to especially high loads due to hot gas, and allow for optimal cooling of these regions.
In a preferred embodiment of the invention, a blade is provided with the at least partially helical cooling channels in the wall of the blade hub. The cooling channel can extend substantially continuously over the entire height of the blade hub, thereby allowing for even cooling of the blade hub in the direction of the blade height. Such a cooling channel also can be produced economically using the method of the invention since it can be drilled in a single working step.
Depending on the thermal load introduced by hot gas flow, it may be advantageous to provide several cooling channels that are separate from each other. For example, several laterally juxtaposed cooling channels can be provided. In the area of the leading edge, three to five of these cooling channels can be provided with their axes arranged substantially parallel to each other in order to always ensure a safe cooling of the corresponding wall section in case of a potential shifting of the stagnation point.
In another embodiment, the cooling channels can be arranged below each other. In this case, the cooling channels do not extend continuously over the entire height of the blade hub, but only over a specific partial section. This makes it possible to account for the thermal load that varies over the blade height, and to provide cooling channels at the places where the thermal load is highest. This configuration also makes it possible to increase the cooling power since the cooling medium is added and removed at several places along the blade height.
In another embodiment several cooling channels can be stacked inside each other. The radial and/or axial offset of the individual cooling channels is selected so that they all extend separately from each other. This allows a strong cooling effect at places with especially high thermal loads without weakening the cross-section of the wall too much.
According to another aspect of the invention, ejection openings can be provided for forming a cooling film. The ejection openings are designed as so-called film cooling holes that start from the cooling channel and end at the surface of the blade hub. A suitable design of the helical extension of the cooling channel makes it possible to achieve an optimum cooling film. The optimum cooling film is also supported by the swirl of the cooling air flow created by the helical shape.
Another important application of the invention relates to providing such a cooling channel in the transition area from the blade hub to the platform. This transition area usually has a transition radius that is subject to very large thermal and mechanical loads. This area therefore must be cooled in a targeted manner in order to not exceed the maximum permissible load values.
It is preferred that the cooling channel is provided with several supply and outlet channels so that the coolant is not excessively heated when flowing through the cooling channel. Such a cooling channel can be created in a simple manner since during the forming of the cooling channel, the helical electrode is positioned so that only one angle sector of one turn extends inside the blade, and the remaining sector is located in the area of a cavity through which the coolant flows. This creates several cooling channel sections located on top of each other, which can be supplied optimally by the coolant flowing in the cavity.